The invention relates generally to adhesives acrylate/methacrylate adhesives initiated by chlorosulfonated polymer. More particularly, the invention relates to such adhesives having a consistent cure profile over their shelf life, and having improved toughening and impact properties at low temperatures.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Acrylate and methacrylate based adhesives are used in the bonding of components, such as in the construction of automobiles, boats, and other products and structures. Typically, two parts of the adhesive are prepared and stored prior to mixing the two parts to give the final adhesive. For acrylate and methacrylate adhesives initiated by chlorosulfonated polymers, one part generally contains one or more initiators, such as the chlorosulfonated polymer (e.g., chlorosulfonated polyethylene), and the other part contains at least one corresponding reducing agent. Generally, the acrylate and methacrylate monomers may be included in both parts.
Typically, the two parts are mixed to apply the adhesive and to polymerize the monomers. During the curing of the mixed adhesive parts, the reaction or polymerization of the acrylate and/or methacrylate monomers is exothermic. Thus, the combined-adhesive parts generally experience an increase in temperature until a peak exotherm temperature is reached. After the adhesive reaches its peak exotherm temperature, the temperature of the adhesive may gradually return to ambient temperature. The cure profile of the adhesive may be characterized, in part, by the value of the peak exotherm temperature, and also by the time to reach the peak exotherm temperature (the peak exotherm time). It should be noted that the peak exotherm temperature and time are generally understood variables in the adhesive industry and are normally readily measurable.
Other adhesive variables of interest, such as the adhesive working time (or open time), adhesive fixture time, and so forth, may relate to the peak exotherm time. For example, the working time typically expires prior to the adhesive experiencing its peak exotherm temperature. In practice, the working time of the adhesive may be characterized as the time from when the two adhesive parts are mixed to the point in time that the combined adhesive parts become difficult to apply or non-malleable, i.e., at the onset of a significant viscosity increase in the adhesive during its curing. An adhesive passing its working time may lose its ability to adhere to the substrates or objects being bonded. As for fixture time, it may fall at various points along the cure profile relative to the peak exotherm time, depending on how fixture time is defined and on the given application, and so on. Certain adhesive manufacturers and users may characterize fixture time as the point in the curing of the adhesive where the adhesion and strength of the final adhesive are such that the bonded objects no longer require external construction supports, for example. However, it should be emphasized that both the working time and fixture time may be defined differently by the various adhesive manufacturers and users. Indeed, working time and fixture time are generally application-dependent. For instance, the working time may be a function of the size of the adhesive bead that is applied to the bonded objects, and so on. The fixture time may depend on the size and weight of the bonded objects, for example, if the fixture time is defined by handling strength.
Typically, it is important for these adhesives to have a reproducible or repeatable cure profile over the shelf life of the adhesive, so that end users may predict the available working time, fixture time, and so forth. For example, the end user may rely on the predicted cure profile to estimate a fixture time to know how long the user should wait before de-molding or de-clamping the bonded objects. In general, the end user may rely on the predicted cure profile to design or modify its application process. The cure profile of the mixed adhesive should be substantially the same, whether the adhesive parts have been stored for one day, one month, or one year, and should be substantially the same as the initial cure profile that may be reported by the adhesive manufacturer or determined by the end user. A repeatable cure profile may be especially important in the bonding of large components, such as in the construction of boats, truck cabs, truck trailers, and other structures.
Again, acrylate and methacrylate-based adhesives that may be used in the bonding of objects, such as in the manufacture of boats and large trucks, are those initiated by chlorosulfonated polymers (e.g., chlorosulfonated polyethylene). Advantageously, acrylate and methacrylate-based adhesives are generally curable at ambient or room temperatures. Further, the chlorosulfonated polymers not only initiate polymerization of the acrylate and methacrylate monomers, but may also act as a polymeric modifier, affecting the physical properties of the cured adhesive. Generally, such adhesives, when cured, exhibit desirable physical properties, such as good lap shear strength at higher temperatures, e.g., about 150° F. to 220° F. (66° C. to 104° C.). However, these adhesives initiated by chlorosulfonated polymer, when cured, may become brittle at low temperatures, e.g., at −40° F. (−40° C.). This is due, in part, to the fact that the chlorosulfonated polymer typically has a glass transition temperature, Tg, higher than about −17° F. (−27° C.). Therefore, it is desirable to improve the performance (e.g., toughening and impact resistance) of such adhesives at low temperatures without sacrificing their performance (e.g., lap shear strength) at high temperatures. Another problem with use of these adhesives is that the cure profile varies throughout the shelf life of the adhesive parts. Thus, with acrylate/methacrylate adhesives initiated by chlorosulfonated polymer, the peak exotherm time and temperature, working time, fixture time, and other properties related to the cure profile, vary undesirably over the shelf life of the adhesive parts.